feat(omaha): add a bootid field

the bootid field will be used by CoreOS to uniquely identify a boot of
an instance.

omaha/omaha.go

@@ -82,6 +82,9 @@ type App struct {
 	// update engine extensions
 	Track       string       `xml:"track,attr,omitempty"`
 	FromTrack   string       `xml:"from_track,attr,omitempty"`
+
+	// coreos update engine extensions
+	BootId       string       `xml:"bootid,attr,omitempty"`
 }
 
 func NewApp(id string) *App {

omaha/omaha_test.go

@@ -28,6 +28,10 @@ func TestOmahaRequestUpdateCheck(t *testing.T) {
 		t.Error("Expected an App Id")
 	}
 
+	if v.Apps[0].BootId != "{7D52A1CC-7066-40F0-91C7-7CB6A871BFDE}" {
+		t.Error("Expected a Boot Id")
+	}
+
 	if v.Apps[0].UpdateCheck == nil {
 		t.Error("Expected an UpdateCheck")
 	}